A. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device, and particularly relates to a semiconductor device manufacturing method concerned with improvement of a surface electrode film of a power semiconductor device for use in a power conversion apparatus or the like.
B. Description of the Related Art
In the background art, aluminum (hereinafter abbreviated to “Al”) or Al-based alloys are often used as materials of electrode films and wires with which the surfaces of semiconductor substrates of power semiconductor devices are coated. This is because Al is a low-resistance material and can be easily processed into an electrode film shape and a wire shape. A sputtering method is often used as an Al film formation method. It is easy to control a composition for forming an Al film using a sputtering method, and this method also is superior in view of mass productivity because film thickness and film quality can be made uniform even in large-diameter semiconductor substrates.
On the other hand, in the surface of any of the aforementioned semiconductor substrates, a large number of irregularities are formed due to insulating film steps formed in boundaries between insulating films and electrode film contact portions in semiconductor function regions, hole-like wire contact portions provided in the insulating films, etc. Further, the Al film to be applied to the surface of the semiconductor substrate is required to be exactly applied to the irregular shapes of the insulating film steps, the holes and so on, and to be formed to contain as few voids as possible, in order to obtain electric properties and reliability thereof.
However, when the Al film is formed in this way on the surface of the semiconductor substrate having irregularities such as insulating film steps, in some aspect ratio the holes or the insulating film steps have, it is often a practical problem that step coverage may lead to disconnection, or in formation of voids even if disconnection can be avoided.
On the other hand, in recent years, power semiconductor devices have often been mounted on vehicles. As for the semiconductor devices to be mounted on vehicles, deterioration in their electric properties due to reduction in their reliability may lead to serious accidents. Therefore, the semiconductor devices are required to have extremely high reliability. The reliability of any semiconductor device is strongly linked to heating due to electricity applied thereto. In particular, current applied to a power semiconductor device is so high that the amount of heat generated is also high. A temperature rise exceeding a temperature rise expected in device design may lead to fatal deterioration of reliability in the semiconductor device. As a solution to this, a double-sided cooling structure instead of a conventional single-side cooling structure has been under review for power semiconductor devices to be mounted on vehicles, such as IGBTs (Insulated Gate Bipolar Transistors), in order to efficiently release heat generated by electricity applied thereto (JP-A-2001-332664 (Abstract, FIG. 1) or JP-A-2005-129886 (Abstract, FIG. 2)). This is because the single-sided cooling structure consisting of a surface side of Al wire connection and a back side of a heat sink cannot be expected to provide efficient heat radiation while the double-sided cooling structure releases heat from the both sides of a semiconductor substrate.
There is a document suggesting improvement of step coverage in such a manner that a metal layer is formed to cover edges of various step portions provided on a semiconductor substrate or acute edges appearing in free ends of holes provided likewise, and afterwards, the metal layer is made obtuse on each edge by dry etching using Ar sputtering and then coated with a conductive layer (JP-T-2000-503806 (FIG. 4)(The form “JP-T” as used herein means a published Japanese Translation of a PCT patent application)). In addition, there is a description that a surface etching step is provided to remove alumina from a surface before an aluminum electrode pattern is formed, and the surface is then patterned (JP-A-7-142479 (FIG. 3)). Further, there is a description that an aluminum-based alloy film must be modified, for example, by isotropic etching to provide a taper in the shape of any contact hole because step coverage over the contact hole is not good in the aluminum-based alloy film (JP-A-10-173049 (paragraph 0053)). Furthermore, there is also a description about a method in which a taper is formed in any contact hole by isotropic etching such as wet etching or plasma etching (JP-A-56-90525 (Detailed Description of the Invention)).
However, as shown in FIGS. 3A-3C, a well-known power semiconductor device such as a trench gate IGBT has semiconductor function regions which are formed on the surface of a silicon substrate 1 according to design conditions of the device and which include required trenches 2, p-type base regions 3, n+-type emitter regions 4, polysilicon gate electrodes 5, gate insulating films 6, interlayer insulating films 7 such as PSG films, contact holes 8 opened in the interlayer insulating films, etc. On the back side, the power semiconductor device has n+-type buffer layer 10, p-type collector layer 11 and collector electrode 12 as shown in FIG. 3C. Al electrode film 9a is applied all over the semiconductor function regions on the surface of the aforementioned silicon substrate 1 by a sputtering method. Thus, Al electrode film 9a is also applied in common onto the pattern of interlayer insulating films 7 on the surface of substrate 1. In the power semiconductor device, each interlayer insulating film 7 is applied thickly to be about 0.2 μm to 1.0 μm thick, and further Al electrode film 9a is also applied thickly to be about 0.5 μm to 5 μm thick. However, when the semiconductor substrate surface where contact holes or interlayer insulating film steps are formed as described above is coated with such a thick Al electrode film 9a, voids are often formed to be open in the surface of Al electrode film 9a. Particularly, as shown in FIG. 3B, such a void 9x is apt to be formed on a recess portion such as contact hole 8 or an insulating film step. The occurrence of void 9x is a problem peculiar to thick Al electrode film 9a. When a large number of voids 9x are formed thus in Al electrode film 9a, the conductivity to be expected as an electrode film deteriorates, leading to lowering in reliability as a uniform film. Further, as shown in FIGS. 4A and 4B, when a plurality of voids 9x are open in the surface of Al electrode film 9a, the voids are expanded as shown by reference numeral 9y in FIG. 4A or 9z in FIG. 4B due to permeation or survival of various treatment liquids or cleaning liquids used in a wafer process which is a post step so that there is problem in occurrence of secondary contamination.
In the double-sided cooling structure described in JP-A-2001-332664 (Abstract, FIG. 1) or JP-A-2005-129886 (Abstract, FIG. 2), a metal electrode plate which can serve as a heat sink is connected also onto a surface-side electrode film by soldering or the like. It is therefore necessary to laminate a solderable metal film such as nickel (Ni) film 13 further onto Al electrode film 9a as shown in FIG. 4B. In the step where nickel (Ni) film 13 is laminated onto Al electrode film 9a, voids open in the surface of Al electrode film 9a may be etched into more widely open voids. The voids cannot be fully filled with Ni particles entering the voids, but the voids may be deformed into the aforementioned voids 9z. Further, even if Al electrode film 9a is not judged as defective at that time, Al electrode film 9a may deteriorate and turn defective in a long-term reliability test.